The present invention relates to diagnostic technique for an evaporative emission control system, and in particular to diagnostic systems and methods for detecting leakage in an evaporative emission control fluid circuit for a vehicle.
An evaporative emission control system mounted on a vehicle is designed to prevent the escape of gasoline vapors to the atmosphere by using a canister filled with activated carbon or charcoal. While the engine is out of operation, the fuel vapors are directed through tubing to the canister, and the activated carbon or charcoal adsorbs the fuel vapors. When the engine starts, a purge line or passage is opened under a predetermined engine operating condition, and fresh air is drawn through the canister by the action of engine vacuum. The flow of the fresh air removes the fuel vapors from the carbon and carries the fuel vapors to the intake passage downstream of the throttle valve, so that they are burned in the engine.
A Japanese Patent Kokai Publication No. 7(1995)-139439 discloses a diagnostic system for performing a leak diagnosis to detect leakage in such an evaporative emission control system.